One of the important parameters characterizing laser diodes is the divergence of the output beam. The narrower the divergence, the brighter is the laser source, the easier it is to focus laser power and the more power that can be coupled into an optical fiber for fiber delivery of the laser power. In semiconductor diode lasers, divergence of the beam perpendicular to the epitaxial layers is defined by epitaxial layer refractive index profile design. Divergence of the beam in a plane parallel to the epitaxial layers of multi-mode broad area lasers is defined by the lateral shape of the multiple optical modes.
There are two basic structures which can provide lateral optical confinement in semiconductor lasers. One is a prior art gain-guided structure illustrated in FIG. 1. Semiconductor laser 100 comprises a substrate 101, on which a sequence of epitaxial layers is grown, starting with a n-cladding layer 102, followed by a n-waveguide layer 103, an active layer 104, a p-waveguide layer 105, a p-cladding layer 106 and finally a cap layer 107. The active layer 104 may consist of one or more sub-layers such as quantum wells.
Current I is injected into the semiconductor laser 100 through a stripe contact or electrode 108 of a certain width W1. At currents higher than the laser threshold current Ith, an optical gain g(I) is achieved inside a pumped region under the stripe contact of the semiconductor laser structure. A complex refractive index N inside the pumped region becomes different from that of surrounding unpumped regions. The contrast in the complex refractive index thus created provides a lateral optical confinement in the gain-guided lasers.
Gain-guided lasers are easy to fabricate by deposition of blocking dielectric layers, ion implantation, or patterned metal contacts but they have relatively higher threshold currents and lower efficiencies.
Another way of providing lateral optical confinement is with an index-guided laser structure, such as in the example of FIG. 2a. A lateral profile of refractive index is introduced during laser fabrication by selective etch and regrowth of material with different index or selectively changing the index by vacancy induced or impurity induced disordering. Index-guided lasers have lower threshold and higher efficiency.
In FIG. 2a, semiconductor laser 200 comprises a substrate 201, on which a sequence of epitaxial layers is grown to form an optical waveguide with a perpendicular refractive index profile. The sequence starts with a n-cladding layer 202, followed by a n-waveguide layer 203, an active layer 204, a p-waveguide layer 205, a p-cladding layer 206 and finally a cap layer 207. The active layer 204 may consist of one or more sub-layers such as quantum wells.
The lateral profile of refractive index is formed under an index-guided stripe 209 of width W2 by lowering the index of refraction in one or more of the various epitaxial layers, which are part of the optical waveguide. For example, in the top four layers 204-207, contiguous regions 204′-207′, respectively, of lower refractive index are formed by one or more of the fabrication processes mentioned above.
Thus, lower index active regions 204′, lower index p-waveguide regions 205′, lower index p-clad regions 206′, and lower index cap regions 207′ constitute a confining region with a lower refractive index which provides the lateral optical confinement for guiding an optical beam in a lateral direction.
Various methods and structures have been applied to create lateral refractive index profiles for improving lateral guiding of lasing beams within semiconductor lasers.
Ashby (U.S. Pat. No. 4,965,806) teaches a semiconductor laser devices in which a lateral refractive index profile is induced by thermally heating regions adjacent to an active region to compensate for an effect of junction heating during operation.
This approach has a serious practical limitation for high-power lasers because the attendant problem of heat removal from the junction is further aggravated by the need to extract the additional heating used to thermally adjust the lateral refractive index profile.
Agrawal (“Lateral Analysis of Quasi-Index-Guided injection Lasers: Transition from gain to Index Guiding”, J. of Lightwave Technology, vol. LT-2, No 4, pp. 537-543, 1984) describes a transition from gain-guided to index-guided mode of operation in lasers with current stripe and index stripe of the same widths. Since for both gain-guided and index-guided modes of operation the stripe width remains essentially the same, there is no practical improvement in the lateral beam divergence of the laser.
Itoh et al. (U.S. Pat. No. 4,144,503) discloses a laser structure, in which a Fabry-Perot cavity is embedded within a light guide.
In FIG. 2b (corresponding to Itoh's FIG. 2(c)), a plan view of a mesa part shows an active region 2 formed in the shape of a stripe 20 μm wide and 300 μm in length, with insulating film 8 surrounding all four side faces of the mesa part to separate it from a surrounding light guide part 11. Thus the index-guiding and gain-guiding stripes coincide.
Apart from the difficulty of fabricating buried optical mirrors of high quality for the Fabry-Perot cavity, Itoh's structure has the additional limitation that the insulating film 8 substantially increases the thermal resistance of the mesa part. The increased thermal resistance limits the maximum optical power at which the laser may be operated.
On the other hand, LaComb (U.S. Pat. No. 6,256,330) teaches an index guided semiconductor laser possessing both index tailoring and gain tailoring. In a Fabry-Perot cavity formed between cleaved facets, lateral optical confinement forming a lateral waveguide is created by a ridge. In the ridge structure, a step in refractive index is provided by the dielectric material deposited on the sides of the ridge, as well as the surrounding air. Semiconductor index of refraction values generally fall in the range from 3.0 to 4.0, whilst dielectric material has substantially lower refractive index values in the approximate range of 1.5 to 2.2, and air has practically 1.0, similar to the value in vacuum.
Current confinement which basically determines gain guiding can in theory be accomplished through selective doping of a p-type material in the n-type layers making up the waveguide. However, a parasitic p-n junction thus formed in the ridge provides a current leakage path, effectively increasing the width of the current path and laterally spreading the gain guiding to approximately the width of the ridge or even wider.
This structure also has thermal limitations in that the ridge constricts thermal flow, thereby substantially increasing the thermal resistance. The increased thermal resistance limits the maximum optical power at which the laser may be operated.